Charlemagne's coinage reforms took place in a large part from 771 with standardised obverses, and really took off in 793 or so, where a heavier silver coin was established, with standardised obverses and reverses. These dates, and especially the latter 793 establishment of Charlemagne's novus denarius, are contemporary to efforts in Southern England towards coinage reform. However, the coinage of England and that of Francia were not the same currency, rather their use of roughly identical size and silver content allowed them to be freely exchanged without conversions. In England, these reforms took place under the reign of Offa of Mercia. Two aspects of Offa's reign are important to understanding the coinage changes: its extent within England, and the simultaneous growth of England in international trade.

Within England, the eighth century had been one of consolidating power south of the Humber. Æthelbald of Mercia in the first half of the century subjected all of southern England besides Wessex to Mercian power. After a two year interlude, Offa rose as king of Mercia in 757, and the following decade saw the general reestablishment of Æthelbald's territorial extent. This Mercian hegemony spanned onwards past Offa as well, certainly through the reign of Cenwulf (796-821) and arguably for Ceolwulf I after that (821-823). It is easy to look at Southumbrian England at this time as a contiguous kingdom ruled firmly by Mercian kings. Charters of the period would seem to back this, with numerous styling Offa as "rex totius Anglorum" (king of all of England), and a couple naming Æthelbald "rex Suthanglorum" (king of the South Angles). This is an overly simplified view, though, and for the lion's share of charters, Æthelbald and Offa are simply referred to as kings of Mercia. For Offa, most charters with some "rex anglorum" variation are spurious tenth century forgeries. Rather than a unified polity across Southern England, then, the kingdom of Mercia was a quiltwork of sub-kings client to the kings of Mercia.

Here the charter evidence offers more solid footing. Several charters from the 770s are from Offa to Ealdred, who is explicitly called the sub-king of the Hwicce (a minor kingdom centred around Worcester and Winchcombe, long tied to Mercia). Even more interesting is a charter from the reign of Cenwulf, where lands granted to Christ Church Canterbury by King Egbert of Kent are restored after "Offa, king and glory of Britain, transferred the possession of these lands, and distributed them to his thegns, saying that it was wrong that his thegn [King Egbert] should have presumed to give land allotted to him by his lord into the power of another without his witness. " Clearly, Egbert at this time was in some sort of subservient role to Offa. A simple consideration of the location of many of Offa's land grants from charters, which range across England well beyond the traditional upper Thames territory of Mercia, show the extent Offa exerted influence on client courts. An important document as well is the Tribal Hidage, generally thought to be of Mercian origin. It outlines the size of Mercia and other kingdoms, and very well could be a list for estimating tribute needs from each of these kingdoms. Various dates have been offered for the document, but due to its mention of Mercia as 30,000 hides (an Anglo-Saxon unit of land), a late eighth century date seems most likely, since Mercia proper did not seem to reach that size until the tail end of Æthelbald's reign.

So, the political landscape of eighth century England, to be very concise, was one of three kingdoms: Northumbria, Wessex, and Mercia. There are exceptions to this rule; Wessex was intermittently under Mercian rule, while Kent was intermittently rid of Mercian rule, but at large this is the case. Of these, Mercia is the clear dominant power, and may even have exerted some influence in Wessex and Northumbria after the cementing of marriage ties during Offa's reign.

Concurrent to this was a rise in international trade in England. For centuries there had been contact with the continent, and Frankish goods can be seen especially in the South-East long before the eighth century (and within that particularly in Kent). These trade connections reached new levels, though, seen well in the rise of wics. The term Wic, roughly translated to a mercantile area or emporium, saw new use with the development of the towns of Hamwic (near modern Southampton) and Ipswic (Ipswich). These new trade centres show rapid growth as the eighth century progressed, to the point of rivalling more established sites like Lundenwic (London) or York (possibly Eoforwic). Besides this development, royal interest in trade seems to be growing. A major concern for Mercian expansion southeast of its landlocked heartland seems to have been for access to the trade connections of London. An even more concrete example can be seen in a letter from Charlemagne to Offa from 796, where explicit discussion of merchants and trade is the bulk of the letter. Charlemagne promises to Offa "you have also written about merchants, and we allow that they shall have protection in our kingdom," and later makes stipulations on ongoing trade, saying "as for the black stones which your reverence begged to be sent we will willingly order them to be given. But as you intimated your wishes concerning the length of the stones, so our people make a demand about the size of the cloaks, that you may order them to be such as used to come to us." The recognition of what are presumably English merchants in Francia, and the general importance of trade in this letter, point to the connections England was forging with the Continent throughout the eighth century.

For trade to reach such intense levels, currency generally becomes necessary. There was coinage in Britain well before the reign of Offa, in the form of sceattas. These were highly individualised silver coins, struck in local mints with diverse designs. While kings may have taken an interest in the production of sceattas as a revenue source, there is nothing to identify them as royal coinage. These sceattas roughly aligned with silver levels of Continental coins, but diversity on both sides of the Channel make comparison difficult. Charlemagne's and Offa's respective coinage reforms change the currency environment drastically. Rather than sceattas of various designs and provenance, Offa's pennies bear largely similar designs, with a portrait obverse and symbol (usually a cross) on the reverse. They bear the inscription Offa Rex, and are of a standardised silver ratio and size. Albeit none of these are completely new concepts. As mentioned, sceattas did roughly coincide with Merovingian and other Continental coinages in weight and silver. Also, coins bearing king's names do slightly predate Offa's own. However, what really is remarkable about Offa's coinage is its standardisation on such a wide scale: it is estimated anywhere from two to ten million coins were struck. For such massive production, a level of administration must have entered coin production the likes of which did not exist before. The coins even take on an importance beyond their monetary use. To be able to standardise coinage across an area as wide as the kingdom of Mercia would be a powerful tool to consolidate power and spread authority. It is telling that major mints at Hamwic and London were outside of the Mercian heartland, showing Mercian interest across Southern England. The dominance of these coins, and their clear royal provenance, would make them a daily reminder of Mercian strength in markets around southern England.

Considering all this, coinage and eighth century English politics and relations are deeply intertwined. The size and duration of Mercia was formidable, and perhaps it took a kingdom of such extent to be able to attempt major coin reforms. Likewise, the growth of merchant activity would only be improved by the protections that a large and relatively longstanding kingdom could provide (just as Charlemagne promised Offa in his 796 letter). With the concurrent rise of Carolingian Francia, and Charlemagne's standardisation of coinage therein, maintaining currency parity would have been an economic necessity for the English. Under Offa, the circumstances were right for this to occur on a widespread and massive scale.

Sources:

James Campbell, The Anglo-Saxons, chapter 5 (this whole book is a good overview of Anglo-Saxon England by the way)

P. Grierson and M. Blackburn, Medieval European Coinages: Volume I, the Early Middle Ages, 'The Carolingians'

R. Naismith, Money and Power in Anglo-Saxon England: The Southern English Kingdoms, 757–865

C. Hart, 'The Tribal Hidage,' Transactions of the Royal Historical Society 5:21

D. Whitelock, English Historical Documents volume 1, c. 500-1042, no. 80, no. 197

The electronic Sawyer (online catalogue of Anglo-Saxon charters)

There were still nominally seven kingdoms, though most had come under the hegemony of the midland realm of Mercia. Mercian overlordship decined after king Offa's death in 796 and the southwestern kingdom of Wessex emerged supreme in 825, though political unification would take another century. Mercian pennies were minted in Kent and East Anglia under Offa's overlordship, and production was resumed under West Saxon domination, extending also to Wessex proper, so monetary unification may have occurred by c.850, extending to Northumbria perhaps a couple of decades later.